Various materials have been reported as electrochromic (hereinafter may be abbreviated as “EC”) materials of which optical absorption properties (colored state and light transmittance) are changed by an electrochemical oxidation-reduction reaction.
Though metal oxides such as WO3 are known as inorganic EC materials, the method of forming a film from such a metal oxide is limited to, for example, deposition, which disadvantageously prevents formation of a large-size film.
Organic EC materials are described in, for example, PTL 1 disclosing an EC device including a conductive polymer and PTL 2 disclosing an EC device including an organic low-molecular-weight compound such as viologen.
The conductive polymer described in PTL 1 can be directly formed as an EC layer on an electrode by electropolymerization of a monomer. As the conductive polymer that forms the EC layer, for example, polythiophene, polyaniline, and polypyrrole are known.
In these conductive polymers, the π-conjugated chain length of the main chain is changed by electrochemical oxidation or reduction of the polymer. This causes a change in the electronic state of the highest occupied molecular orbital (HOMO). As a result, the absorption wavelength of the conductive polymer is changed.
These conductive polymers in electrically neutral states absorb light in the visible light region. They, therefore, have colors. Oxidation of the conductive polymers shifts the absorption wavelengths to the longer wavelength side.
The absorption in the visible light region disappears by the shift of the absorption wavelength to the infrared region side, and thereby the EC device is bleached.
In the EC material of the viologen-based compound described in PTL 2, dications are dissolved in a solution in the bleached state, and viologen is converted into a radical cation state by a reduction reaction to deposit on an electrode and to have a color.
In PTL 1, though the stability is increased by delocalizing the unstable radical cation in its molecule, the stability is insufficient. Accordingly, repetition of oxidation and reduction deteriorates the material to decrease the performance of the EC device.
Furthermore, the conductive polymers absorb visible light in the electrically neutral state, i.e., the polymers have colors. Accordingly, if there is a portion in which the electrochemical reaction for discoloring is insufficient, remnant occurs to make it difficult to give high transparency.
The viologen-based EC organic compound in PTL 2 repeats deposition and dissolution and thereby causes a deterioration phenomenon.
This deterioration phenomenon is believed to be caused by insolubilization owing to irreversible crystallization or polymerization and to cause “remnant” in which transparency is not obtained even in the state for bleaching.
In addition, the viologen-based EC organic compound generates an unstable radical cation when reduced. Since the viologen-based EC organic compound does not have a mechanism for stabilizing the radical cation in the molecule, the radical cation is unstable. Accordingly, the durability of the device is low.